1. Field of the Invention
This invention relates to surface analysis, and, more particularly, to ion-scattering spectrometry.
2. Description of the Related Art
The technique of ion-scattering spectroscopy typically involves the bombardment of a surface by energetic primary ions during which the energy of the scattered ions is analyzed. Ion-scattering spectrometry (ISS) can be divided into three categories depending on the energy of the incident ion beam: high energy or Rutherford backscattering spectrometry (1-2 MeV), medium energy (100-400 keV), and low energy (0.5-10 keV). Together these three ranges are capable of providing information about specimen surfaces at depths ranging from the outermost atomic layers to a few micrometers.
Typically, measurements are performed by bombarding the surface with a mono-energetic beam of collimated noble gas ions and then determining the energy spectrum of the ions scattered typically at a fixed angle, usually equal to or greater than 90.degree.. Since the scattering process can be treated as a simple binary collision, it can be shown from conservation of energy and momentum considerations that the relationship between the mass of an elastically scattered ion M.sub.p and the mass of a target atom M.sub.t for a scattering angle of 90.degree. is given by: ##EQU1## where E.sub.1 and E.sub.0 are the energies of the scattered and incident ions, respectively. For instance, for the scattering of helium, the energy spectrum becomes a mass scale, making it possible for conventional ISS to identify all elements except hydrogen and helium.
For low energy ISS, the variation of sensitivity with atomic mass is generally less than one order of magnitude, and detection limits are on the order of 10.sup.-2 to 10.sup.-3 monolayers. The only important energy loss is due to binary collisions. This leads to a very simple spectrum for low energy ion-scattering where the energy loss is directly related to the ratio between the mass of the bombarding ion and the mass of the scattering atom. Low energy ISS yields information only about the outermost atomic layer, since ions that penetrate that layer are generally neutralized by electrons in the solid and are subsequently not passed by conventional energy analyzers. Depth information is generally obtained by repeated analysis, such that the bombarding ions are allowed to sputter away layers of the surface and expose succeeding layers to analysis. Alternatively, an ion-scattering spectrometer may be provided with an auxiliary sputtering ion gun for the removal of surface layers.
Ion-scattering spectroscopy is one of the most rapidly developing techniques in surface science today because it complements diffraction techniques because, in ion-scattering spectroscopy, a classical particle (an ion) and simple classical concepts ("shadowing" and "blocking") are used. A repulsive scattering potential leads to a region behind each atom into which no ion can penetrate. This region is called a shadow cone and atoms located inside the con of another target atom cannot contribute to the scattering process. Atoms that are either scattered or recoiled from a surface can also be deflected by neighboring surface atoms. These deflections result in blocking cones about neighboring atoms which tend to limit atom ejection at specific angles. The angles and the energies E.sub.1 and E.sub.2 following a collision event can be expressed in terms of an impact parameter p, which is the distance of closest approach of the projectile and target atom if no scattering occurred. Ions with a small impact parameter p are scattered through large angles while ions with large p are only slightly deflected. This gives rise to the shadowing and blocking cones. Analytical formulas have been developed for calculating the dimensions of shadowing and blocking cones in binary collisions. See, e.g., Surface Sci., 141, 549 (1984).
As a result of using a classical particle and classical concepts, ion-scattering spectroscopy provides direct information on the relative positions of atoms in a surface region, although it is generally difficult to analyze a surface atomic structure fully by this technique alone. One of the most significant problems with ISS as an analytical tool is that they employ magnetic or electrostatic analyzers. These types of analyzers detect scattered ions which are only a small fraction of the total scattered particles. Scattered neutrals are not detected. Therefore, the technique suffers from poor sensitivity.
Moreover, ISS is a destructive technique because relatively high ion doses are required to generate the ion flux needed for detection. Conventional ISS usually requires potentially damaging ion doses (approximately 10.sup.15 ions per square centimeter) to obtain a spectrum since (1) the technique detects only ions and disregards neutrals which often constitute more than 90% of the scattered flux, and (2) single channel devices, such as electrostatic energy analyzers, are typically used for data collection. Buck and coworkers have shown that both of these shortcomings can be overcome by using (1) a multiplier that is sensitive to both neutrals and ions, and (2) a pulsed beam with time-of-flight (TOF) analysis which collects particles of all energies concurrently in a multi-channel mode.
Aono and coworkers have demonstrated a technique called impact collision ion-scattering spectroscopy (ICISS) for analyzing the structure of surface atomic vacancies including the displacement of surrounding atoms. ICISS also analyzes the concentration and chemical activity of surrounding atoms, including the geometry of chemisorbed species. Phys. Rev. Letters, 49, 567 (1982). ICISS is a specialized form of conventional low energy ion-scattering spectroscopy with respect to the experimental scattering angle. The scattering angle is chosen to be close to 180.sup..about. so that the impact parameter p is nearly zero. Therefore, scattered ions that have made head-on collisions against target atoms are observed. The most striking characteristic of ICISS is that the ion-scattering in this specialized condition "sees" just the center (or the close vicinity of the center) of each target atom because of the small value of the impact parameter p.
As previously mentioned, an atom in an ion beam forms a shadow called a shadow cone into which no incident ion can penetrate, and any atom concealed by this shadow cone does not contribute to ion-scattering. By virtue of the characteristic mentioned above, ICISS can determine the shape of the shadow cone and the atomic geometry of surfaces quantitatively using such shadowing effects among the surface atoms. Stated another way, the backscatter mode of ICISS eliminates the blocking phenomenon observed in conventional ISS leaving only the shadowing effect, and, thus, simplifies the analysis. The ICISS technique detects only ions and cannot separate atomic structure effects from electron neutralization effects. Therefore, the data is ambiguous. Aono and coworkers did, however, demonstrate that it was possible to obtain electron density distributions above surfaces using ion-scattering spectrometry.
Alkali metal ions have been used in ion-scattering spectrometry in place of the noble gas ions that are most commonly employed as the incident beam. In 1984, Niehus demonstrated that alkali metal ions could be substituted for noble gas ions to improve the sensitivity of ICISS. The low ionization potential of the alkali metals means that more of the incident ions survive the collision with the surface as ions, i.e., a smaller fraction of the incident ion flux is neutralized in the collision with the sample surface. This leads to higher sensitivity for conventional ion-scattering spectrometers which detect only charged species. Unfortunately, when this technique is used, a significant number of the impinging alkali metal ions deposit on the sample surface, and, thus, contaminate it. Moreover, like conventional ISS, the signal is determined solely by the scattered ion flux, so the technique cannot be quantitative.
Aono and coworkers demonstrated that ion-scattering spectrometry could be used to gain information on the spatial distribution for surface electrons, i.e., surface electron densities. Because Aono and coworkers were detecting only ions, neutralization effects in the spectra were superimposed on the atomic structure effects. These various effects could not be separated to provide accurate analysis. Aono and coworkers obtained information on electronic distributions by measuring how the scattered ion yields change as angles were varied. However, if only ions are detected and if there are changes in the intensities of the detected ions, ICISS cannot determine if the changes in the ion intensities come from changes in electron neutralization probabilities, from atomic structure effects, or from a combination of the two. Therefore, ICISS cannot separate atomic structure an electron density contributions to the ion-scattering yield. But this work did demonstrate that it was possible to get electron density distributions above surfaces (60-100% versus less than 20% for noble gas ions).
At present, the only known energy analysis method which detects both ions and neutrals is the time-of-flight analyzer. Unfortunately, time-of-flight analyzers commonly have relatively poor resolution compared to electrostatic and magnetic analyzers. However, the resolution of a time-of-flight analyzer may be improved by providing a longer flight path length. Providing a sufficiently long flight path for a time-of-flight ion-scattering spectrometer is difficult because it significantly increases the total evacuated volume of the instrument. This poses both fabrication and pumping problems.
In 1984, Buck and coworkers demonstrated that the time-of-flight technique could be used to get very high sensitivity in ion-scattering spectrometry by detecting of both ions and neutrals using a detector which is sensitive to both ions and fast neutrals, such as a channel electron multiplier. See, Surface Sci., 141, 549 (1984). This technique eliminated the problem of not knowing how much neutralization occurred at the sample surface and rendered the technique quantitative. This technique was also used to obtain atomic structure analysis of surfaces. Only scattering rather than recoiling was used however.
For the purposes of this disclosure, the term "recoil" refers to phenomenon involving dislodged surface species, and the term "scattering" refers to reflection of the primary ion beam. Both recoiling and scattering may involve ions as well as neutrals, but most commonly recoiled species will be neutrals and scattered species will be ions.
In 1987, van Zoest and coworkers in Holland showed that a time-of-flight analysis of scattered and recoiled particles, which detected the neutrals and the ions, could be used to obtain information on atomic structure. See Surface Sci., 109, 239 (1981). However, the path length of the instrument used in these studies was relatively short and the resolution was insufficient to discriminate recoiled and scattered particles.